In terms of enhancing yield and reliability of semiconductor device manufacturing processes, inspection techniques for detecting defects in the surfaces of wafers used as substrates for semiconductor devices have increasingly become very important. Ideally, no defects are formed in wafer surfaces; however, some defects are negligible in terms of device characteristics and yield in device manufacturing. Therefore, the wafer surfaces are examined based on predetermined criteria and the wafers are determined whether they are conforming products or defective products.
In this specification, crystal defects, flaws, foreign matters, etc. in wafer surfaces are collectively referred to as “defects”. The defects include pits, crystal defects such as COPs, and unevenly polished portions and scratches formed due to machining as well as particles which are foreign matters attached to wafer surfaces. Further, the term “wafer surfaces” herein refers to both a main surface on the front side of a wafer and a main surface on the rear side thereof, and is distinguished from only a surface on one side.
Conventionally, using a light point defect (LPD) inspection apparatus, wafer inspection is performed in which wafer surfaces having been finished by mirror polishing are scanned with laser light thereby detecting scattered light resulted from particles, scratches, and the like in the surfaces. Further, in order to determine the presence and absence of defects that are hardly determined by an LPD inspection apparatus, appearance inspection is also performed in which wafer surfaces are examined by visual observation. Since appearance inspection is an organoleptic test, variation in the determination depending on inspectors cannot be avoided, and it takes time for inspectors to master the examination technique. Therefore, there is a demand for establishing an objective inspection method and an automatic inspection method.
To address the above challenge, we have previously proposed in JP 2010-103275 A (PTL 1), as a wafer inspection method, a method of properly evaluating wafers without appearance inspection especially focusing on defects on the rear surface side of the wafer surfaces. Specifically, the method is a method of evaluating the rear surface of a wafer, including: a mapping step of consecutively taking partial images of the rear surface of a wafer in the circumference direction of the wafer and synthesizing the taken partial images to compose a full image of the rear surface of the wafer; and a differentiation step of differentiating the full image to create a differentiated image of the rear surface of the wafer, wherein the wafer is evaluated by detecting unevenly polished portions, haze, scratches, and particles based on the full image or the differentiated image.